When preparing commercial printing inks, pigments are utilized in a variety of forms. Previously, dry pigments were used exclusively, and they are still being used for formulating certain printing inks such as those used for gravure printing. Dry pigments are generally prepared by filtering the pigment from a water or solvent medium followed by washing, drying and pulverizing to obtain the dry pigment in the desired finely divided or powdered form. Such finely divided or powdered dry pigments are, however, somewhat difficult to redisperse in printing ink vehicles.
Although wet presscakes or water dispersions have been used commercially in the manufacture of printing inks, such feed materials are limited to aqueous systems. The so-called "flushing" procedure has been used extensively for many years. In general, flushed pigments are made by transferring pigment particles from wet presscakes or from an aqueous phase to an organic vehicle such as a non-polar oil or resin phase. Although the flushing has the advantage of avoiding the costly and time-consuming drying operations necessary for the preparation of dry pigments, it has the drawbacks of still requiring special filtration and mixing equipment as well as overcoming the problems attendant upon the use of such equipment.
U.S. Pat. No. 4,634,471 discloses admixing an aqueous pigment slurry with an emulsion containing water, an emulsifying agent and an organic liquid selected from organic resins, organic solvents and mixtures thereof, followed by flushing the admixture thus formed with a hydrophobic organic vehicle. However, after the mixing step and prior to flushing, the admixture must be treated in a "concentration zone" that is preferably a filter such as a plate and frame filter press. Thus, this patented process requires a special filtration step to remove water.
There also have been proposals for preparing offset inks by directly converting the pigment from an aqueous slurry to a finished ink. However, such a process requires a high shear, high horsepower mixer as well as other special equipment.
Prior to the present invention, gravure inks were manufactured using pigments in their dry color form. Thus flushed pigments which offer numerous technical and economic advantages over dry color pigments generally could not be used to prepare inks suitable for gravure printing because of the particular requirements of gravure printing. Other types of inks such as offset inks are manufactured from pigments dispersed in vehicles typically consisting of alkyd polymers in one or more relatively high-boiling point hydrocarbon oils (e.g. Magie Oil 500). However gravure inks utilize pigment vehicles in which the metal resinates are dissolved in a relatively low-boiling hydrocarbon solvents (e.g. toluene, lactol spirits, etc.). Although dry color pigments can be dispersed in such hydrocarbon solvents, the same cannot be said for pigments attempted to be flushed from the aqueous phase (i.e. wet presscake) into the hydrocarbon phase.
Gravure ink vehicles contain a varnish component which is usually a metal resinate or mixture of metal resinates. However, it has also been discovered that some metal resinates produce gravure inks which exhibit poor printing quality.
The present invention provides an effective process for overcoming the problems referred to above. The process is economical and allows for the manufacture of high quality gravure inks containing flushed pigments as well as the traditional gravure ink vehicle components. By practice of the present invention, not only is a high quality gravure ink obtained, but considerable savings are realized because the conventional filtration, drying and grinding steps required to produce dry color are eliminated. Moreover, the process of the present invention can be carried out in an ordinary reactor or tank provided with a low energy agitator rather than the expensive, special high energy equipment required for dispersion of pigments using pigments in the dry color form.